[fea-rs] Add CompilerError::display_verbose

fea-rs collects extensive diagnostic information during parsing and
compilation, and is able to report errors associated to specific
locations in the input FEA. This wasn't very fine-grained, though, and
was causing problems with crater (because sometimes the stderr text for
a given error would be extremely long, and include a bunch of ANSI
escape codes) and so I turned off this finer-grained reporting.

It would be nice to have it available as an option, though, especially
now that I'm trying to add support for new syntax, and would like to see
what the actual errors are.

This adds a new method to the CompilerError that will print the error in
the old verbose style, and makes it so that when you call the fea-rs
binary directly (which is really not useful for debugging) it uses this
method when printing the returned error.

fea-rs/src/bin/compile.rs

@@ -6,7 +6,7 @@ use clap::Parser;
 use fea_rs::{
     compile::{
         self,
-        error::{FontGlyphOrderError, GlyphOrderError, UfoGlyphOrderError},
+        error::{CompilerError, FontGlyphOrderError, GlyphOrderError, UfoGlyphOrderError},
         Compiler, MockVariationInfo, NopFeatureProvider, Opts,
     },
     GlyphMap,
@@ -80,8 +80,8 @@ enum Error {
     MissingGlyphOrder,
     #[error("Error parsing axis info: L{line}, '{message}'")]
     BadAxisInfo { line: usize, message: String },
-    #[error("{0}")]
-    CompileFail(#[from] compile::error::CompilerError),
+    #[error("{}", .0.display_verbose())]
+    CompileFail(#[from] CompilerError),
 }
 
 /// Compile FEA files

fea-rs/src/compile/error.rs

@@ -1,5 +1,7 @@
 //! Error types related to compilation
 
+use std::fmt::Display;
+
 use write_fonts::{read::ReadError, BuilderError};
 
 use crate::{parse::SourceLoadError, DiagnosticSet};
@@ -58,6 +60,26 @@ pub enum CompilerError {
     WriteFail(#[from] BuilderError),
 }
 
+impl CompilerError {
+    /// Return a `Display` type that reports the location and nature of syntax errors
+    pub fn display_verbose(&self) -> impl Display + '_ {
+        struct Verbose<'a>(&'a CompilerError);
+        impl std::fmt::Display for Verbose<'_> {
+            fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+                write!(f, "{}", self.0)?;
+                let diagnostic = match self.0 {
+                    CompilerError::ParseFail(x)
+                    | CompilerError::ValidationFail(x)
+                    | CompilerError::CompilationFail(x) => x,
+                    _ => return Ok(()),
+                };
+                write!(f, "\n{}", diagnostic.display())
+            }
+        }
+        Verbose(self)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;